“Using taxpayer-funded resources to pay staffers to dig up dirt on political opponents isn’t just an ethics violation, it’s a federal crime,” Citizens for Responsibility and Ethics in Washington Executive Director Melanie Sloan said in a statement that explained the complaints filed with both the FBI and the Senate Ethics Committee.

The allegation is that legislative assistants were working on government time for political purposes, conducting opposition research against potential Democratic challenger Ashley Judd. As CREW itself acknowledges, the campaign has said that the individuals in question conducted the research on their own time.

While the lines between official work and campaign work are clear, many staffers engage in both types of activity, being careful to use non-government cellphones and other devices away from the Capitol grounds when doing campaign activities, whether paid or as volunteers. That's exactly what McConnell's office has said.

“The transcript of the recording currently being examined by the FBI — pursuant to a criminal investigation of illegal wiretapping — contains an inaccuracy that partisans have used in an attempt to smear the staff of Senator McConnell. The audio of the presenter refers to members of Senator McConnell’s staff who volunteered “in their free time.” The transcript that was used reads instead, “thank them three times,'" said McConnell spokesman Robert Steurer in a statement. "Although this error may not have been intentional, it is really sad that there are partisan operatives willing to ignore the potential of a crime and focus their attacks on the common practice of young people volunteering for a cause they believe in.”

McConnell's camp also alleges that CREW is politically motivated, pointing to a different Mother Jones report that suggests the watchdog group participated in liberal strategy session that, in part, included targeting McConnell for defeat in 2014.

The CREW complaint follows up on the McConnell campaign's announcement that it is working with the FBI and the U.S. attorney's office in Louisville to try to figure out how the audio recording of internal campaign deliberations was made.